This invention is directed to a new construction for a tundish used in the continuous casting of molten metal, such as for example in the production of steel.
A continuous steel caster is a machine that transforms or casts molten steel into solidified semi-finished shapes such as billets, blooms and slabs. Specifically, molten steel from a steelmaking furnace, usually a basic oxygen furnace, is poured from a ladle through a ceramic shroud into a trough-shaped container or vessel called a tundish, which functions as a continuous reservoir for the mold, as discussed hereinafter. The walls of the tundish consist of a steel shell that has been lined with a refractory material, a portion of which is replaced after each use of the tundish. Specifically, the tundish is lined with a permanent lining that is replaced after approximately 500 casts. Additionally, there is a replaceable lining that is replaced after each cast.
In the operation of the caster, molten steel from the reservoir of steel in the tundish is then poured into a device which sizes and shapes the steel, this is known as the caster mold. As the partially solidified steel emerges from the mold, the steel is in the form of a strand and is moved through cooling devices by an array of motorized rollers. Thus, a continuous supply of molten steel from the tundish to the caster mold will allow steel to be cast without interruption.
In order to protect the molten steel in the tundish, a flux/slag layer thereover is provided for a multiplicity of purposes. For example, such layer functions to (1) maintain the temperature of the molten steel which is typically about 2800.degree. F., (2) protect the molten steel against excessive oxidation, and (3) scavenge undesirable elements from the molten steel.
One problem that arises when molten steel is poured from the ladle into the tundish is that the molten steel rebounds against the tundish bottom and travels in an upward direction along the lining of the tundish walls creating a turbulent flow pattern on the surface of the molten steel bath in the area around the shroud. One of the undesirable effects created by this turbulent flow is that the slag and flux covering the molten steel bath are dispersed. As a result, a portion of the molten steel bath in the area around the shroud becomes exposed to the ambient atmosphere and interacts with oxygen creating inclusions in the molten steel, which may be entrapped and carried over into the solidified steel.
In an attempt to eliminate the undesirable effects created by such turbulent flow, several proposed solutions have been considered. One possible solution involves adding more flux to the molten steel bath in order to prevent the molten steel from becoming exposed to the ambient atmosphere in the area around the shroud. Although additional flux may prevent such exposure, it has also been found that adding more flux causes some of the flux to become entrained in the molten steel, i.e., the flux becomes a part of the molten steel as it is poured into the caster mold and will remain in the molten steel during the solidification process. Another proposed solution was to enclose completely the region above the molten steel bath in the caster tundish and provide an inert gas environment in the enclosed area. Although this eliminates the need for the use of fluxes, the procedure can be technically difficult to use and maintain, and cannot be applied to all tundish designs.
A third possible solution involved the use of elaborate dam and weir systems to reduce the turbulent flow. Such a solution is taught in U.S. Pat. No. 4,177,855. The tundish construction described therein incorporates an impact pad to define the pouring area for the molten metal, and a pair of spaced-apart pivotal refractory, heat-insulating slabs. Such slabs pivot from a generally vertical position at the initiation of the metal pour, and are moved to a horizontal position as the molten metal fills the tundish.
A second type of tundish construction is described in U.S. Pat. No. 4,042,229. The tundish contains a molten metal impact area having a pair of upstanding, spaced-apart beams or weirs. Above such beams, the tundish is provided with a comparable pair of upper beams adjacent the top. As described in such patents, the dams and weirs are oriented in the tundish in a direction transverse to the long axis of the tundish and affect primarily the longitudinal flow of the molten steel into the tundish. The transverse flow of the molten steel is unaffected by these dams and weirs and as a result, the turbulent flow is not eliminated.
Consequently, there is still a need for a tundish which is designed to eliminate the turbulent flow and the undesirable effects created thereby when steel is poured into the tundish. The following specification contains a description of a tundish construction which is effective in achieving the desired result.